Magnetic tape is effectively used to store digital data. Data is written onto the magnetic tape by a write head. The write head converts a current signal containing the digital information into flux patterns which are written as field transitions onto the magnetic tape. The data is retrieved when the magnetic tape is passed by a read head. The read head output may be amplified and equalized before a detector extracts a binary signal containing the data from the read signal. Magnetic tape storage may be modeled as a magnetic recording channel including the effects of converting a binary signal into magnetic tape field transitions and producing a read signal by sensing the field transitions.
One problem with magnetic tape is dropout or the decrease in the read signal amplitude envelope. Dropout is frequently caused by defects in the magnetic tape. These defects may weaken magnetic field transitions on the tape. Defects may also increase the distance between the magnetic tape and read and write heads. In addition to a decrease in the read signal amplitude envelope, a reduction in high frequency components due to dropout changes the shape of pulses in the read signal making correct detection of data more difficult.
Many designs have been proposed to deal with read signal dropout. For magnetic tape holding analog signals such as voice and video, information lost due to dropout may be replaced with substitute information. The substitute information may be obtained from other channels, may be found by averaging information prior to or following the dropout, or may be artificially generated. Provided the dropout duration is not excessive, human perception will not detect that substitute information has been provided. Substitution techniques will not work with recorded digital data, however, since each bit must be either correctly received or reconstructed using error correction schemes.
Other designs for dropout include circuitry to detect when a dropout is occurring. The output of such dropout detection circuitry is a binary signal that may be used to change filter characteristics or to signify that an error has occurred. Dropout detection circuitry may be complex and the resulting filter modification may not occur rapidly enough to prevent data loss.
Another design uses an adaptive filter preceding the detector. Filter parameters are calculated using an error signal based directly on the detector output. Detection is accomplished using the decision feedback equalization method which may result in infinite error propagation.
What is needed is signal dropout compensation that can continuously and adaptively adjust to both the decreasing amplitude and high frequency attenuation occurring during dropout. Dropout compensation should not require complex dropout detection circuitry. For increased effectiveness and applicability to a wide range of tape systems, dropout compensation should be based on magnetic recording channel properties.